Re: ADT Lib: Persistence

[Michael van Acken <mvacken@t-online.de>]

Here is my current idea for the syntax of externalized objects.  Most
of it is from ETH report 156, plus Stewart's suggestion about
provisions for `Alien' objects as in Oberon/F.

file     = {object}.
object   = NUM(ref) [ objdescr ].
objdescr = module type 
           LONGINT(offset) {data object LONGINT(offset)} data.
module   = {char} 0X.
type     = {char} 0X.

`ref' is a unique reference number assigned to an object when it is
externalized.  The first occurrence of a reference number is followed
by the object's qualified type name, and the object's value.

`data' is a sequence of bytes (possibly empty), produced by calling
`Store' on a object.  The sequence does not contain any embedded
objects.

`offset' is an integer number noting the offset from the rider
position just after the offset data, to the next occurrence of an
object embedded in the file.  If it is non-negative, it refers to the
position an object that is referenced by the current object, if it is
negative, its absolute value minus 1 refers to the position of the one
after the current object.

The values for `offset' are inserted by back patching.  If a piece of
`data' is written, its preceding `offset' value is set to point after
the data.  This means that data of this format cannot be sent over a
pipe or a TCP stream without storing it in a file first.  Because the
`offset' values are only needed to support `Alien' objects, it might
be useful to define a second format, that omits the `offset' data
altogether.  The two formats could be distinguished by prefixing the
object sequence with a format id, and could be selected by an
additional parameter of `ConnectReader'.  Does any one second this
proposal?


While all of this is pretty straightforward, I have my problems with
this procedure:

"PROCEDURE (VAR r: ObjectReader) TurnIntoAlien (cause: INTEGER)
This causes the currently Internalized store to become Alien."

See Stewart's mail for an  example how this procedure would be used.
Does anyone have a brilliant idea how we can implement it in a simple
way? 

-- mva